Rivers, lakes, streams, and springs have provided humans and other animals with welcome and plentiful sources of drinking water for many thousands of years. It is well known that as the human population has increased, so too has the demand for drinking water. The Romans, for example, built aqueducts to provide bath water. As the population increased, the water was diverted for drinking and cooking purposes. More recently drilling tools have been used to access drinking water stored underground in aquifers. Not unlike rivers, lakes, and streams, aquifers too have been subject to draw down and pollution often making them unfit as a source of drinking water without extensive and impractically expensive treatment.
More recently, the oceans of the world have garnered increased attention as vast reservoirs of potential drinking water. Due to the relatively high salinity of seawater, the phrase “Water water everywhere, but not a drop to drink” is a truism. Raw seawater contains so much salt, that it is not fit for human consumption.
In the open oceans of the world, seawater contains about 35,000 mg/l of total dissolved solids, also referred to as salinity, slightly less in the polar and coastal regions due to incursion of fresh water from melting ice and runoff from rivers and streams. Substantially reduced salinity from the outfall of the Columbia River in Washington State has been measured more than 200 miles from its mouth. On the other hand, the Persian Gulf and Red Sea are known, for example, to have salinity levels of about 40,000 mg/l due to high rates of evaporation and relatively little rainfall.
It is also generally accepted that to be of drinking quality water cannot possess more than 500 mg/l of total dissolved solids of which chloride content should not exceed 250 mg/l. To meet this standard, over 98.5% of the salt in seawater must be removed. Potable water is defined herein as water of drinkable quality containing no more than 500 mg/l of total dissolved solids of which chloride does not exceed 250 mg/l.
Over 70 elements are dissolved in seawater. However, only six ions make up over 99% by weight of all the dissolved solids in seawater. Sodium and chloride, each of which are in the form of monovalent ions, not only account for its salty taste, but also make up slightly more than 85% of all the dissolved solids in seawater. Sodium accounts for about 30% and chloride accounts for slightly more than 55% by weight of all dissolved solids in seawater. The other four ions include calcium, magnesium, potassium and sulfate. Calcium, magnesium, and sulfate are divalent ions. Potassium is a monovalent ion accounting for only about 1.1% by weight of seawater.
A process known as reverse osmosis has been applied to produce potable water from seawater. Reverse osmosis is a process that is reversed from the naturally occurring process of osmosis. Osmosis occurs when solutions of differing concentrations are separated by a semi-permeable membrane. Hydraulic pressure is exerted across the membrane upon the solution of higher solute concentration by the solution of lower solute concentration. The osmotic pressure across the membrane is directly proportional to the difference in concentration between the two solutions. Pressure must be applied to the more concentrated solution to counteract the natural osmotic pressure being exerted upon it. To reverse the direction of the natural osmotic flow, additional pressure is required.
Reverse osmosis (RO) requires that high pressure be applied to the seawater due in part to the fineness of the membrane and due also to the additive force required to overcome the osmotic pressure preferring to work in the opposite direction to encourage the dilution of seawater rather than an increase in salt concentration. A reverse osmosis membrane is extremely fine and capable of rejecting extremely small contaminants such as bacteria, sugars, proteins, dyes, and salts. Under such high pressure, some potable water permeates through the semipermeable membrane, leaving the balance of the seawater and nearly all the salt retained without passing through the membrane forming a more salt-rich concentrate to be swept away under pressure. Thus, the seawater is fractionated by the membrane into a permeate of low salt concentration and a concentrate or retentate of salt concentration higher than seawater.
Feasibility of desalinating seawater has been known. However, the high material and operational costs to support seawater desalination facilities have stymied broad application of the process.
It is known in the art that about 360 psi must be applied to seawater at 35,000 mg/l salt just to neutralize the natural osmotic pressure exerted by potable water from the opposite side of the membrane. It is further disclosed in the art that total pressure of about 800–1,200 psi is required to produce, through an RO membrane, potable water from seawater.
Application of such high pressure requires expensive and high pressure tolerant materials including stainless steel to contain the pressurized saline solutions. Moreover, expensive pump machinery and high energy required to operate the machinery to produce the high pressure required drives up costs significantly. Moreover, application over time of such force to a reverse osmosis membrane tends to rapidly reduce its efficiency through compaction effects. These effects become increasingly severe above 600 psi reducing flow across the membrane over time and correspondingly reducing membrane life adding further to costs of operation including downtime and replacement of membranes.
Reduced pressures required for filtration through RO membranes have also been disclosed in the art. For example, U.S. Pat. No. 4,156,645 to Bray, the disclosure of which is herein incorporated by reference, discloses an RO desalination process that includes filtration in two stages, the first stage producing an intermediate product water containing from one fourth to one half the salt content of the seawater feed at a pressure of 300 to 550 psi through a relatively loose RO membrane, followed by a subsequent stage filtration of the intermediate product through a tighter RO membrane at the same or greater pressure than the first stage producing a final product water containing less than 2,000 ppm total dissolved solids. It is also disclosed in Bray that such lower pressures allow for use of lower cost plastic materials.
Reverse osmosis membranes are known generally in the art to be the tightest of all membranes. However, additional less restrictive filtration mechanisms are known in the art. Ultrafiltration is generally known to include filtration of particulates with molecular weights of 1,000 to 100,000 g/mol. Operating loosely between and to some extent overlapping these two levels of restriction is nanofiltration (NF). As disclosed in U.S. Pat. No. 5,587,083 to Twardowski, the disclosure of which is herein incorporated by reference, like RO membranes, NF membranes are composite structures composed of cross-linked aromatic polyamides comprising a microporous polymer sheet supporting a skin layer. Such a membrane structure is generally known as a thin film composite (TFC). Unlike RO membranes, however, NF membranes have a larger pore size in the skin layer and a net negative electrical charge inside the individual pores. The pores work to exclude material by size and the electrical charge works to exclude electrically charged species according to the surface charge density of introduced ions. The '083 patent discloses employment of nanofiltration to selectively separate polyvalent ionic species such as sodium sulfate from aqueous solutions also containing, for example, monovalent species such as sodium chloride, the sodium chloride passing through the nanofiltration membrane. The '083 patent also discloses that reverse osmosis rejects all ionic species.
Separation of salts on an electrical charge basis utilizing nanofiltration membranes is also disclosed in U.S. Pat. No. 5,458,781 to Lin, the disclosure of which is herein incorporated by reference. The '781 patent proposes a process to separate the monovalent anion bromide from sea and brackish waters utilizing a combination of RO and NF membranes. It is disclosed that the NF membrane selectively allows bromide ions to pass while rejecting polyvalent anions. It is proposed that the charge separation properties of the nanofiltration membrane disclosed therein may provide for separation of a feed stream into a monovalent anion rich permeate stream leaving behind polyvalent anion rich retentate.
Similarly, U.S. Pat. No. 6,190,556 to Uhlinger, the disclosure of which is herein incorporated by reference, proposes use of RO and NF membranes in combination with each other to produce potable water from seawater. It is disclosed therein that a vessel containing both membranes receives seawater pressurized to 250–350 psi for filtering in series first through at least one NF membrane the permeate stream from which is interrupted and pressurized via the same pump to 250–350 psi and pumped through at least one RO membrane, producing a permeate of potable water and a concentrate that is used to flush the NF membrane. It is proposed therein that the NF membrane produces a permeate stream of substantially reduced divalent ions, about 95% reduction, and moderately reduced monovalent ions, about 50% reduction. It is proposed therein that the salinity of the first permeate stream should be about 10,000 to 15,000 mg/l due in no small part to the presence to a great degree of monovalent sodium and chloride in seawater. The RO membrane of the second stage is able to operate under lower pressures due to the reduced salt content of its feed stream. Had the RO membrane been used in the initial stage to receive seawater feed at 35,000 mg/l salt, the existence of the extra salt would require substantially more operating pressure to obtain a permeate.
There still remains, however, a need for a simple, cost efficient and practical means for continuous production of potable water from seawater without use of reverse osmosis membranes.